1. Field of the Invention
The invention relates generally to a method for removing metal or mineral deposits from surfaces, in particular, from surfaces of drilling machinery in the oil industry.
2. Background Art
Subterranean oil recovery operations may involve the injection of an aqueous solution into the oil formation to help move the oil through the formation and to maintain the pressure in the reservoir as fluids are being removed. The injected water, either surface water (lake or river) or seawater (for operations offshore) generally contains soluble salts such as sulfates and carbonates. These salts may be incompatible with the ions already contained in the oil-containing reservoir. The reservoir fluids may contain high concentrations of certain ions that are encountered at much lower levels in normal surface water, such as strontium, barium, zinc and calcium. Partially soluble inorganic salts, such as barium sulfate (or barite) and calcium carbonate, often precipitate from the production water as conditions affecting solubility, such as temperature and pressure, change within the producing well bores and topsides. This is especially prevalent when incompatible waters are encountered such as formation water, seawater, or produced water.
Some mineral scales have the potential to contain naturally occurring radioactive material (NORM). The primary radionuclides contaminating oilfield equipment include Radium-226 (226Ra) and Radium-228 (228Ra), which are formed from the radioactive decay of Uranium-238 (238U) and Thorium-232 (232Th). While 238U and 232Th are found in many underground formations, they are not very soluble in the reservoir fluid. However, the daughter products, 226Ra and 228Ra, are soluble and can migrate as ions into the reservoir fluids to eventually contact the injected water. While these radionuclides do not precipitate directly, they are generally co-precipitated in barium sulfate scale, causing the scale to be mildly radioactive.
Because barium and strontium sulfates are often co-precipitated with radium sulfate to make the scale mildly radioactive, handling difficulties are also encountered in any attempts to remove the scale from the equipment. Unlike common calcium salts, which have inverse solubility, barium sulfate solubility, as well as strontium sulfate solubility, is lowest at low temperatures, and this is particularly problematic in processing in which the temperature of the fluids decreases. Modern extraction techniques often result in drops in the temperature of the produced fluids (water, oil and gas mixtures/emulsions) (as low as by 5° C.) and fluids being contained in production tubing for long periods of time (24 hrs or longer), leading to increased levels of scale formation. Because barium sulfate and strontium sulfate form very hard, very insoluble scales that are difficult to prevent, dissolution of sulfate scales is difficult (requiring high pH, long contact times, heat and circulation) and can only be performed topside.
When pipes and equipment used in oilfield operations become layered with scale, the encrustation must be removed in a time- and cost-efficient manner. Occasionally, contaminated tubing and equipment is simply removed and replaced with new equipment. When the old equipment is contaminated with NORM, this scale encrusted equipment cannot be disposed of easily because of the radioactive nature of the waste. The dissolution of NORM scale and its disposal can be a costly and hazardous affair. At present, a considerable amount of oilfield tubular goods and other equipment awaiting decontamination is sitting in storage facilities. Some equipment, once cleaned, can be reused, while other equipment must be disposed of as scrap. Once removed from the equipment, several options for the disposal of NORM exist, including canister disposal during well abandonment, deep well injection, landfill disposal, and salt cavern injection.
Typical equipment decontamination processes have included both chemical and mechanical efforts, such as milling, high pressure water jetting, sand blasting, cryogenic immersion, and chemical chelants and solvents. Water jetting using pressures in excess of 140 MPa (with and without abrasives) has been the predominant technique used for NORM removal. However, use of high pressure water jetting generally requires that each pipe or piece of equipment be treated individually with significant levels of manual intervention, which is both time consuming and expensive, but sometimes also fails to thoroughly treat the contaminated area. When scale includes NORM, this technique also poses increased exposure risks to workers and the environment.
While chemical chelants, such as EDTA (ethylenediaminetetraacetic acid) or DTPA (diethylenetriaminepentaacetic acid), have long been used to remove scale from oil field equipment, once EDTA becomes saturated with scale metal cations, the spent solvent is generally disposed of, such as by re-injection into the subsurface formation. However, because the process requires that disposal of the solvents once saturated, the large amounts of a fairly expensive solvent necessary for decontamination renders the process economically prohibitive.
U.S. Pat. No. 5,234,602 discusses a process whereby the chelating agent is regenerated in solution throughout the decontamination cycle. The '602 patent teaches that by lowering the pH of the solution to a pH of 4-9, preferably 5-7, following the sequestration of barium by DTPA, the chelated barium ions may be displaced from the chelating agent and precipitated as an insoluble barium salt, such as barium sulfate. Once the precipitant has formed and has been removed from the DTPA solution, the DTPA solution may be reused to dissolve additional scale. FIGS. 1-2 of the '602 patent show that while the cumulative amount of barium sulfate removed from a tubular can be increased using the regenerated DTPA, the amount removed per cycle actually decreases. The observed decrease in productivity of the DTPA solution may result from increased levels of impurities, i.e., insoluble salts formed from the other mineral deposits on the equipment or the successive addition of the acid and base, in the solution with each successive cycle and/or a reduction in the concentration of the chelating agent as more water is formed during the regeneration cycle.
Accordingly, there exists a need for an economically efficient means for removing scale from oilfield equipment with a low risk of exposure to radioactive materials.